Exchange scaling of ultrafast angular momentum transfer in 4f antiferromagnets

Y W Windsor,C Krellner,V Borisov,L Rettig,S.-E Lee,K Kliemt,C Schüßler-Langeheine,D Zahn,D V Vyalikh,A Ernst,O Eriksson,U Staub,D Thonig,N Pontius

doi:10.1038/s41563-022-01206-4

Abstract

Ultrafast manipulation of magnetism bears great potential for future information technologies. While demagnetization in ferromagnets is governed by the dissipation of angular momentum1–3, materials with multiple spin sublattices, for example antiferromagnets, can allow direct angular momentum transfer between opposing spins, promising faster functionality. In lanthanides, 4f magnetic exchange is mediated indirectly through the conduction electrons4 (the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction), and the effect of such conditions on direct spin transfer processes is largely unexplored. Here, we investigate ultrafast magnetization dynamics in 4f antiferromagnets and systematically vary the 4f occupation, thereby altering the magnitude of the RKKY coupling energy. By combining time-resolved soft X-ray diffraction with ab initio calculations, we find that the rate of direct transfer between opposing moments is directly determined by this coupling. Given the high sensitivity of RKKY to the conduction electrons, our results offer a useful approach for fine tuning the speed of magnetic devices.

Highlights

Lanthanides are increasingly important in technology because their 4f spin moments reach exceptionally large sizes compared to those of 3d transition metals
RKKY acts as a Heisenberg exchange, with a coupling energy expressed as[5] j ∝ |I|2χ, in which χ is the non-local susceptibility of the conduction electrons and I is the on-site exchange integral between the 4f states and the conduction electrons[6]
While reports of ultrafast 4f spin dynamics in antiferromagnets are scarce[8–10], one experiment performed on an antiferromagnetic (AF) lanthanide suggested the existence of this channel, which has been proposed as a route to overcome speed bottlenecks associated with the lattice[9]

Summary

Introduction

Lanthanides are increasingly important in technology because their 4f spin moments reach exceptionally large sizes compared to those of 3d transition metals. To account for the varying moment sizes, and given the universal dynamics observed, we facilitate a more direct comparison of the demagnetizations by considering angular momentum transfer rates α, in units of μB ps–1 (exact definition in the Methods).

Results

Conclusion